In traditional systems for drilling boreholes, rock destruction is carried out via rotary power conveyed by rotating the drill string at the surface using a rotary table or by rotary power derived from mud flow downhole using, for example, a mud motor. Through these modes of power provision, traditional bits such as tri-cone, polycrystalline diamond compact (“PDC”), and diamond bits are operated at speeds and torques supplied at the surface rotary table or by the downhole motor.
In some circumstances and under some drilling conditions when using these traditional techniques, the drilling rate (or rate of penetration, “ROP”) may be compromised. When that occurs, the operator has several options to improve the drilling rate. The operator can trip out the drill string for a new drilling assembly more likely to be successful in drilling under the existing circumstances. Alternatively, if a rotary table on the surface provides the drilling power, the operator can change the rotary speed within a relatively narrow range, such as approximately 60 to 250 revolutions per minute (“RPM”). If the drilling system includes a downhole positive-displacement motor (“PDM”), the operator can change the motor speed over a range, for example, of approximately 150 RPM to approximately 300 RPM (for a medium speed 6¾-inch motor). A change in motor speed, however, can produce proportionate flow rate changes that can have a profound effect on hole cleaning, pressure drop, and other factors. As yet another alternative, the operator can attempt to adjust the weight on bit by adjusting the hook load at surface.
In all of these techniques the operator is remote, both in distance and time, from the changing bottom hole conditions that caused the compromised ROP. As a consequence, it may take some time for the compromised ROP to manifest itself at the surface and for the operator to recognize that the ROP has decreased. In addition, the operator's response actions, such as adjusting the rotary speed, hook load, or flow rate, are equally remote from the bit on bottom. Various load factors such as torque and drag may attenuate the operator's control action and compromise its effectiveness.
Continuous movement, including rotation, of the drill string has important benefits in addition to transferring power to the bit. Torque and drag consumption along the drill string due to frictional losses may reduce the weight and rotary torque available to be transferred to the bit, which may cause the power available at the bit to be variable or unpredictable. This power variability may, in turn, compromise ROP. An important source of frictional loss is static friction, which typically occurs during non-rotary periods, momentary stoppages of the pipe during sliding due to stick/slip, and periodic stoppages during additions of drill pipe. In addition to the static friction, an immobile pipe string is more likely to become differentially stuck due to pressure differential between the hole and the formation. Further, pipe rotation is known to keep the cuttings mobile and off the bottom of the hole, especially in horizontal wells.